Gonadotropin releasing hormone (GnRH) is the primary hypothalamic regulator of gonadotropin biosynthesis and secretion in the anterior pituitary gland. Activation of the GnRH receptor causes rapid increases in phosphoinositide hydrolysis and calcium mobilization, and is followed by biphasic increases in cytoplasmic calcium ([Cal)i and LH secretion in gonadotroph-enriched pituitary cells. The initial phase of LH release depends largely on mobilization of intracellular calcium by Ins(1,4,5)p3, and the sustained phase of hormone secretion requires continuous entry of extracellular calcium through plasma-membrane channels. The agonist-stimulated entry of calcium during the plateau phase of the cytosolic calcium response depends on calcium entry through both L-type and dihydropyridine-insensitive channels. Patch clamp analysis of the calcium currents in rat gonadotrophs revealed the presence of rapidly inactivating T-type channels and a delayed component with slower inactivation, corresponding to L-type channels. The latter are potentiated and inhibited by dihydropyridine agonist and antagonist analogs, and participate in agonist-mediated calcium influx and gonadotropin secretion. Depolarization-induced calcium entry through L-channels initially potentiates the agonist-induced release of calcium from intracellular stores, but attenuates the subsequent plateau phase of the response. The retention of an attenuated LH secretory response when the plateau phase of the cytoplasmic calcium response was abolished in the absence of extracellular calcium indicates that while influx and elevation of intracellular calcium are predominant factors in GnRH-induced hormone secretion, other elements of the calcium-phospholipid signaling system also contribute to the maintenance of LH release during agonist-receptor interaction. Protein kinase C, which was previously shown to participate in LH synthesis and secretion, was found to activate voltage-and dihydropyridine-sensitive calcium channels in cultured pituitary cells, but to exert inhibitory effects on cytoplasmic calcium levels during prolonged activation by phorbol esters. Thus, protein kinase C exerts both positive and negative control of the function of voltage-sensitive calcium channels during its participation in the mechanism of hormone secretion in pituitary gonadotrophs. The existence of additional regulators of gonadotropin secretion, acting via calcium-dependent mechanisms, was shown by findings on the abilities of GABA and endothelin to stimulate LH release from cultured gonadotrophs. Activation of GABA-A receptors promotes calcium influx through L channels, secondary to depolarization by increased chloride conductance. Endothelin also stimulated gonadotropin release, primarily by activating phosphoinositide hydrolysis with increased calcium mobilization and influx through L-type voltage sensitive calcium channels. These factors may serve as additional hypothalamic regulators of gonadotropin secretion from the pituitary gland.